Why does our porta potty smell like ammonia in November cold?

That ammonia smell is mostly urine load, not the cold killing your blue. Ammonia comes from urine breaking down in the holding tank, and one unit for 25 guys on a weekly pump fills up with a lot of it by day four or five. The cold does play a part though. Our units on gravel sites always got worse in fall because the vent stack pulls air out by the sun warming it, and on gray 30 degree days that draft basically stops. So the smell stays in the box instead of going up the pipe.

A few things that fixed it for us:

• Check the count. OSHA 1926.51 Table D-1 says 20 or more workers need 1 seat and 1 urinal per 40 workers, so 25 guys means a unit with a urinal at minimum. If yours has no urinal, ask for one that does or add a second unit.
• Look at the vent cap. Leaves or a bent cap kill airflow.
• Ask the driver to bump the deodorizer charge or add a midweek service visit instead of switching products.

In my experience the midweek pump is what actually made it go away.